This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for Secondary Battery and the Fabrication Method Thereof earlier filed in the Korean Industrial Property Office on Mar. 5, 2001 and there duly assigned Serial No. 2001-11152.
1. Field of the Invention
The present invention relates to a secondary battery and more particularly a secondary battery having improved safety vent and gasket for attaining securely sealed battery, and a fabrication method thereof.
2. Description of the Related Art
Lithium batteries are capable of charging and discharging and are classified into nickel-cadmium (Nixe2x80x94Cd) batteries, nickel-hydride (Nixe2x80x94MH) batteries, and lithium secondary batteries. Specifically, the lithium secondary batteries are rapidly developing in view of their excellent energy density per unit weight compared to the nickel-cadmium (Nixe2x80x94Cd) batteries or nickel-hydride (Nixe2x80x94MH) batteries. The lithium secondary batteries can be classified into liquid electrolyte batteries and solid electrolyte batteries according to the electrolyte used. In general, batteries using liquid electrolyte are referred to as lithium-ion batteries and batteries using polymeric electrolyte are referred to as lithium polymer batteries.
A conventional lithium secondary batteries includes a battery unit mounted inside a can. The battery unit has a positive electrode plate and a negative electrode plate with a separator disposed therebetween.
A cap assembly is connected to the upper portion of the battery unit. The cap assembly includes a cap cover, a current controller, a safety vent and a cap plate interposed by means of an insulating member, sequentially stacked. The cap assembly has a gasket installed at its outer periphery to be insulated from the can.
Here, the positive electrode tap is drawn from the positive electrode plate through an insulation plate and its ends are welded to the cap plate. The cap plate contacts the safety vent having a concave portion where explosion occurs first due to formation of a notch.
The battery is constructed such that the above-described parts are sequentially stacked and then a portion between the safety vent and the gasket is forcibly crimped to obtain a sealed space so as to prevent leakage of electrolytic solution.
The conventional battery encounters with the following problems.
First, the positive electrode tap has its end welded by laser welding or ultrasonic welding at the bottom surface of the central portion of the cap plate to be electrically connected with the end of the cap plate. In the event of abnormal welding, heat is transferred to the safety vent to produce voids, by which a perfectly sealed space cannot be established, degrading the reliability of the battery.
Second, the gasket installed along the outer periphery of the safety vent, has ends bent toward its inner part. Crimping is made in a state in which the bent outer periphery is secured on an inner circumference of the gasket.
In the course of performing the above-described processes, a gap may be created due to adhesion inferiority during the crimping between the outer periphery of the safety vent and the inner circumference of the gasket. Accordingly, an organic electrolyte may leak through the gap, due to an increased internal pressure of the battery leading to imperfect sealing of the battery.
Therefore, it is necessary to attain safety of the battery by timely explosion of the concave portion of the safety vent so that gas generated by the increased internal pressure due to abnormal operation of the battery can be exhausted. However, if a perfectly sealed space is not provided due to generation of voids or gap, the concave portion cannot be ruptured at an appropriate pressure. As a result, the battery may be exploded due to rapidly increasing internal pressure due to instantaneous exothermic reaction occurring inside the battery.
It is therefore an object of the present invention to provide a secondary battery having an improved connection structure of a safety vent and a gasket in order to attain perfectly sealed space in the battery, and a fabrication method thereof.
It is another object of the present invention to provide a secondary battery having an improved structure at a fixing portion of a positive electrode tap and a cap plate.
It is yet another object to increase the production efficiency of making a secondary battery by reducing the number of fabrication processes.
It is still yet another object to reduce the thickness of portions of a secondary battery.
To accomplish the above and other objects, there is a secondary battery including a battery unit having a positive electrode plate, a negative electrode plate and a separator interposed there between, a can for accommodating the battery unit, and a cap assembly connected to the upper portion of the can and having a cap cover, a safety vent installed in the lower portion of the cap cover and being ruptured at a predetermined internal pressure of the battery, and a gasket provided along the periphery of the safety vent, where the end of the safety vent is bent inwards to be filled with the gasket provided along the outer periphery of the safety vent, so that the safety vent is fixedly inserted into the gasket.
The safety vent is preferably bent inwards from the periphery of a base having a notch and the outer periphery of the end is positioned on the inner circumference of the gasket.
Also, the gasket preferably wraps upper and lower surfaces of the end by a connecting portion integrally extending from the bottom surface of the gasket, to form an alternately stacked structure.
Further, the safety vent preferably has an extension tap drawn from its end, the extension tap being connected to the positive electrode tap connected to the positive electrode plate.
Preferably, the extension tap is positioned at the periphery of the can to be welded to the positive electrode tap.
Also, the extension tap is preferably integrally formed with the end.
According to another aspect of the present invention, there is provided a method of fabricating a secondary battery including a battery unit, a can for accommodating the battery unit, and a cap assembly having a cap cover, a safety vent and a gasket sequentially positioned on the can, the method including the steps of bending an end of the safety vent ruptured at a predetermined internal pressure of the battery toward the center of the safety vent, injecting a raw material of the gasket into the outer periphery of the safety vent to fill upper and lower surfaces of the bent portion so that the gasket is integrally fixed to the safety vent, and compressing the safety vent, the gasket and the can.
Alternatively, the present invention provides a method of fabricating a secondary battery including a battery unit, a can for accommodating the battery unit, and a cap assembly having a cap cover, a safety vent and a gasket sequentially positioned on the can, the method including the steps of forming an end of the safety vent ruptured at a predetermined internal pressure of the battery in a state in which the safety vent is allowed to stand upright, injecting a raw material of the gasket into the outer periphery of the safety vent to wrap the end, bending the end of the safety vent filled with the gasket so that the gasket is integrally fixed to the safety vent, and compressing the safety vent, the gasket and the can.